Those are almost certainly "ghosts" caused by the internal reflection of light from very bright stars. (That is, the light is reflecting off the insides of the camera, filter, etc.) The pattern is an image of the "entrance pupil" of the telescope, which in practice means the primary mirror + the blockage caused by the secondary mirror and ...
BTW if anyone wants a quick and fast query to solution do the following:
Go to https://skyserver.sdss.org/dr12/en/tools/search/sql.aspx. Paste a query like this:
s.specobjid, s.ra, s.dec, s.z
FROM SpecObj as s
s.z > 0 AND s.z < .18 AND s.ra > 0 AND s.ra < 50 AND s.dec > 0 AND s.dec < 30
Then after downloading a csv file ...
SDSS DR12 Catalog Data
looks like a good starting point,
apparently pretty open to those willing and able to figure it out.
site hosts Jupyter notebooks to query CasJobs in SQL.
The Large Scale Structure galaxy catalog under
BOSS value added catalogs
may also be relevant.
What they did was the following: for each individual filter, they assembled overlapping frames into a combined, single-filter image. E.g., they combined several g-band images into a single g-band image, combined several r-band images into a single r-band image, etc. (So they ran SWarp five times, once for each filter.) They then put each combined single-...
The idea of Trottier & Borra is to analyze signals that might be hidden IN the stellar spectrum. I know this sounds weird; it did sound weird to me when I first heard of it too.
You know how you can decompose light in diferent colors, right? Well, we astronomers like to think of colors in terms of wavelength (with $~4500$ Angstroms being blue and $~7000$...